I'm still, sorry, on F12 and sure need to upgrade.
Way back when I first had a bad disk on the "system" disk that one was 80G, I only had a 200G lying around.
Next time that happened my other 200G wasn't "big" enough it said, so I put a 500G in there.
Now it seems I got more bad blocks etc but I need to lower the size 'cos I don't want to put a 1Tb HDD in there.

My question from all of this is, how do I decrease the image so I can put it all on a smaller HDD?
Used space on the system disk's partitions is about 30G, so an 80G disk should be sufficient.

Can anyone tell me how to do this?

What I can think of is that I need to "move" all data to the "beginning" of the HDD, then make an image it but not the entire disk, just the data.
I've tried that with no luck since the image seem to get as big as the HDD, hence why I always needed to increase the HDD all the time.

Although you "can" shrink your partition that's not how I would do it. I would put the 2nd drive in, format it and copy all the data over at the file level (rsync would be good for that). You don't want the file system that you are copying to be active so if it's got your root file system on it you would want to boot from either rescue mode or a Live CD to do the copy from. You'll want to check over your fstab file on the new drive to make sure all the device references are correct for the new drive after copying.

I assume you have a separate /boot partition that you are copying over as well? You want to keep /boot in a relatively small partition (~100-250MB)at the beginning of the drive. You can create the /boot partition first and copy your data over with rsync but yes the first time you boot with the new drive and the old one removed you'll have to boot from alternate media, mount your file systems and chroot onto the new drive and run the "grub-install <device>".

Another question doing this... Isn't there a way to like "refresh" the udev lists of devices?
I added the disk but I had to reboot to get it up in the /dev list.
Been googling this too and they all say:

What kind of drive are you hooking up? I guess you can hot plug a SATA drive but I've never done it. I would shut the system down before even plugging in a SATA drive but that's me. USB is the only thing I would be comfortable hot plugging on low end hardware and USB devices should be detected just fine.

UPDATE2:
Seems there's lots of "files" not getting copied but since they all are under /sys, like /sys/devices and such I figure that is OK... right?

You don't want to copy /sys /proc and all those other file systems. You need to make sure it stays on your root file system which is why I would do it from rescue mode or something similar and mount the partition that you want to copy somewhere other than "/". OR just add the "-x" rsync switch so it doesn't try to copy files outside of the intended file system. You should have 0 errors if you do it right. Redirect the output to a file and open it with an editor if you want to look back on the rsync output.

Don't seem to be able to install grub again.
First it said that it's already installed, so I 'rpm -e' it.
Next it won't be installed cos it's trying to get the files through internet, but I got my own repo on the server just for this cos.
A pal long time ago showed me how to set it up thus all files are rsynced to my server and all installes/updates are taken from that.
Trying to use the NIC just starts the NetworkManager and that doesn't work it seems. And I know that if that is in use something is always making the network not to work.

Can I install grub from the F12 CD I got when in rescue mode?
If yes, then how can I do that.

You weren't trying to install grub to your boot sector using rpm commands were you? The grub package being installed is not the same as installing grub to your boot sector. Sounds like you already had the grub package installed but you use the "grub-install" command as I mentiond before to install grub to your boot sector.